User interface design

Recap• OOD is an approach to design so that design

components have their own private state and operations

• Objects should have constructor and inspection operations. They provide services to other objects

• Objects may be implemented sequentially or concurrently

• The Unified Modeling Language provides different notations for defining different object models

Recap

• A range of different models may be produced during an object-oriented design process. These include static and dynamic system models

• Object interfaces should be defined precisely using e.g. a programming language like Java

• Object-oriented design simplifies system evolution

Objectives

• To suggest some general design principles for user interface design

• To explain different interaction styles• To introduce styles of information

presentation• To describe the user support which should be

built-in to user interfaces• To introduce usability attributes and system

approaches to system evaluation

Topics covered

• User interface design principles• User interaction• Information presentation• User support• Interface evaluation

The user interface

• System users often judge a system by its interface rather than its functionality

• A poorly designed interface can cause a user to make catastrophic errors

• Poor user interface design is the reason why so many software systems are never used

Graphical user interfaces

• Most users of business systems interact with these systems through graphical interfaces although, in some cases, legacy text-based interfaces are still used

GUI characteristics

Characteristic DescriptionWindows Multiple windows allow different information to be

displayed simultaneously on the user’s screen.Icons Icons different types of information. On some systems,

icons represent files; on others, icons representprocesses.

Menus Commands are selected from a menu rather than typedin a command language.

Pointing A pointing device such as a mouse is used for selectingchoices from a menu or indicating items of interest in awindow.

Graphics Graphical elements can be mixed with text on the samedisplay.

GUI advantages

• They are easy to learn and use. – Users without experience can learn to use the system

quickly.• The user may switch quickly from one task to

another and can interact with several different applications.– Information remains visible in its own window when

attention is switched.• Fast, full-screen interaction is possible with

immediate access to anywhere on the screen

User-centred design

• The goal is to sensitise software engineers to key issues underlying the design rather than the implementation of user interfaces

• User-centred design is an approach to UI design where the needs of the user are paramount and where the user is involved in the design process

• UI design always involves the development of prototype interfaces

User interface design process

Executableprototype

Designprototype

Produce paper-based design

prototype

Producedynamic design

prototype

Evaluate designwith end-users

Implementfinal userinterface

Evaluate designwith end-users

Analyse andunderstand user

activities

UI design principles

• UI design must take account of the needs, experience and capabilities of the system users

• Designers should be aware of people’s physical and mental limitations (e.g. limited short-term memory) and should recognise that people make mistakes

• UI design principles underlie interface designs although not all principles are applicable to all designs

User interface design principles

Principle DescriptionUser familiarity The interface should use terms and concepts which are

drawn from the experience of the people who willmake most use of the system.

Consistency The interface should be consistent in that, whereverpossible, comparable operations should be activated inthe same way.

Minimal surprise Users should never be surprised by the behaviour of asystem.

Recoverability The interface should include mechanisms to allowusers to recover from errors.

User guidance The interface should provide meaningful feedbackwhen errors occur and provide context-sensitive userhelp facilities.

User diversity The interface should provide appropriate interactionfacilities for different types of system user.

Design principles• User familiarity

– The interface should be based on user-oriented terms and concepts rather than computer concepts. For example, an office system should use concepts such as letters, documents, folders etc. rather than directories, file identifiers, etc.

• Consistency– The system should display an appropriate level

of consistency. Commands and menus should have the same format, command punctuation should be similar, etc.

• Minimal surprise– If a command operates in a known way, the user should be

able to predict the operation of comparable commands

Design principles• Recoverability– The system should provide some resilience to

user errors and allow the user to recover from errors. This might include an undo facility, confirmation of destructive actions, 'soft' deletes, etc.

• User guidance– Some user guidance such as help systems, on-line

manuals, etc. should be supplied• User diversity– Interaction facilities for different types of user should

be supported. For example, some users have seeing difficulties and so larger text should be available

User-system interaction

• Two problems must be addressed in interactive systems design– How should information from the user be

provided to the computer system?– How should information from the computer

system be presented to the user?

• User interaction and information presentation may be integrated through a coherent framework such as a user interface metaphor

Interaction styles

• Direct manipulation• Menu selection• Form fill-in• Command language• Natural language

Direct manipulation advantages

• Users feel in control of the computer and are less likely to be intimidated by it

• User learning time is relatively short• Users get immediate feedback on their actions

so mistakes can be quickly detected and corrected

Direct manipulation problems

• The derivation of an appropriate information space model can be very difficult

• Given that users have a large information space, what facilities for navigating around that space should be provided?

• Direct manipulation interfaces can be complex to program and make heavy demands on the computer system

Control panel interface

Title

Method

Type

Selection

NODE LINKS FONT LABEL EDIT

JSD. example

JSD

Network

Process

Units

Reduce

cm

Full

OUIT

PRINT

Grid Busy

Menu systems

• Users make a selection from a list of possibilities presented to them by the system

• The selection may be made by pointing and clicking with a mouse, using cursor keys or by typing the name of the selection

• May make use of simple-to-use terminals such as touchscreens

Advantages of menu systems

• Users need not remember command names as they are always presented with a list of valid commands

• Typing effort is minimal• User errors are trapped by the interface• Context-dependent help can be provided. The

user’s context is indicated by the current menu selection

Problems with menu systems

• Actions which involve logical conjunction (and) or disjunction (or) are awkward to represent

• Menu systems are best suited to presenting a small number of choices. If there are many choices, some menu structuring facility must be used

• Experienced users find menus slower than command language

Command interfaces

• User types commands to give instructions to the system e.g. UNIX

• May be implemented using cheap terminals.• Easy to process using compiler techniques• Commands of arbitrary complexity can be

created by command combination• Concise interfaces requiring minimal typing

can be created

Problems with command interfaces

• Users have to learn and remember a command language. Command interfaces are therefore unsuitable for occasional users

• Users make errors in command. An error detection and recovery system is required

• System interaction is through a keyboard so typing ability is required

Command languages

• Often preferred by experienced users because they allow for faster interaction with the system

• Not suitable for casual or inexperienced users• May be provided as an alternative to menu

commands (keyboard shortcuts). In some cases, a command language interface and a menu-based interface are supported at the same time

Natural language interfaces

• The user types a command in a natural language. Generally, the vocabulary is limited and these systems are confined to specific application domains (e.g. timetable enquiries)

• NL processing technology is now good enough to make these interfaces effective for casual users but experienced users find that they require too much typing

Multiple user interfaces

Operating system

GUImanager

Graphical userinterface

Commandlanguage

interpreter

Commandlanguageinterface

Information presentation

• Information presentation is concerned with presenting system information to system users

• The information may be presented directly (e.g. text in a word processor) or may be transformed in some way for presentation (e.g. in some graphical form)

Information presentation

Information tobe displayed

Presentationsoftware

Display

Summary• User Interface• GUI Characteristics (Windows, Icons, Menus, Pointing, Graphics)• User Centered Design• User Interface Design Process• UI Design Principles

– User familiarity– Consistency– Minimal Surprise– Recoverability– User guidance– User diversity

• User System Interaction– How users will interact with the software?– How software will present information to the user?

Reference

• Ian Sommerville, Software Engineering, Chapter 13, 6th Edition